Optical recording medium

ABSTRACT

An optical recording medium and a method of writing, reading and erasing information in an optical recording medium are provided in which the optical recording medium comprises a carrier and a recording layer which contains at least one optically active organic compound, wherein said optically active organic compound is an inherently chiral dissymmetric olefinic chromophore.

FIELD OF THE INVENTION

The invention relates to an optical recording medium comprising acarrier and a recording layer which contains at least one opticallyactive organic compound.

The invention also relates to a method of writing, reading and erasinginformation in such an optical recording medium.

The invention further relates to optically active organic compoundswhich are suitable for use in such an optical recording medium.

BACKGROUND OF THE INVENTION

The optical recording media in question are suitable for writing andreading audio information, video images and data. Said recording mediaare mostly disc-shaped and the writing and reading of information takesplace during rotation of the medium by a light beam emanating from alaser. The projection of the modulated laser beam on the recordingmedium brings about a change of the optical properties of said mediumduring the writing operation, which change can be optically detectedlater on during reading. Known media are those which are based on themagneto-optical or Kerr-effect and the "phase-change" mechanism. Furtherknown media include those which contain photochromic compounds, in whichthe color of the compounds changes upon exposure to light of a specificwavelength. Detection of the change in color takes place by using lightof a different wavelength.

An optical recording medium of the type mentioned in the openingparagraph is described in European Patent Application EP-A 312339. Themedium described therein contains optically active compounds comprisingan asymmetric carbon atom and hydrophobic and hydrophilic groups.Exposure to light of a wavelength of 360 nm causes the aggregationstructure of said compounds to change, as a result of which a changetakes place in the optical rotation. Said change in optical rotation isdetected with linearly polarized light having a wavelength of 700 nm.The recorded information is erased with light having a wavelength of 560nm, as a result of which the optical rotation returns to the originalstate. Erasing can also be carried out by a combined operation ofirradiating the medium with light having a wavelength of 360 nm andheating it to 100° C. Of essential importance is the use of threewavelengths or two wavelengths in combination with a heating step forthe write, read and erase processes, because the information must not beerased during reading. As a result of this requirement, use of the knownmedium is rendered complicated.

SUMMARY OF THE INVENTION

An object of the invention is to provide, inter alia, an opticalrecording medium in which the above-mentioned disadvantage is overcome.A further object of the invention is to provide a method in which thereading and erasing of information is based on a new principle.

According to the invention, this object is achieved by an opticalrecording medium comprising a carrier and a recording layer whichcontains at least one optically active organic compound, which recordingmedium is characterized in that the optically active organic compound isan inherently dissymmetric olefinic chromophore. Such a compoundcontains no asymmetric carbon atom, but obtains its optical activitythrough the fact that the compound itself is chiral, i.e. there are twomirror-image isomers or enantiomers of this compound. Both enantiomersrotate the plane of polarization of linearly polarized light in acounterclockwise or clockwise direction and are termed 1- or d-form,respectively. Such a compound without an asymmetric carbon atom isdefined as being inherently dissymmetric. The term olefinic indicatesthat the compound comprises at least one double bond. The term indicatesthat the chromophore compound absorbs light of a specific wavelength ora specific wavelength range. The chirality of these compounds is theresult of a "twist" in the molecules brought about by the presence of anolefinic π-bond. As a result of said "twist", these compounds have ahelical dissymmetry.

The compound consists of two stable enantiomers, one of which is the1-form and the other the d-form. During the writing of information, saidenantiomers can at least partly be converted into one another(isomerized) by photochemical switching, i.e. by exposure to left-handedor right-handed circularly polarized light. By virtue of saidphotochemical switching between the d- and 1-enantiomers, a rotation ofthe plane of polarization of linearly light is obtained, which effectcan be used to read the recorded information. The recorded informationis erased by circularly polarized light the direction of rotation ofwhich is opposite to that used for writing, or by sufficiently powerfullinearly polarized light. "Direct overwrite" of information which hasalready been recorded, i.e. without a separate erasing step, is alsopossible. The switching between the d- and 1-form is reversible (writingand erasing, respectively) and can be repeated many times. The advantageof an optical recording medium according to this embodiment of theinvention is that the operations of writing, reading and erasinginformation can all be carried out at the same wavelength and, hence,only one type of laser light source is required. Only the polarizationstate of the light used is varied, which can be realized in a simplemanner, for example, by providing a λ/4-plate in the light path orremoving said plate. The chiral compounds exhibit great chiropticeffects, i.e. they have high molar rotations [φ] and ellipticities [0].This means that a small concentration of these chiral compounds in arecord layer yields a useful optical recording medium.

Starting from a racemic mixture, i.e. 50 mol % of 1-enantiomer and 50mol. % of d-enantiomer, an excess of, for example, 1% of one of theenantiomers is formed during writing with circularly polarized light.During erasing, the racemic mixture is formed again. By virtue of thegreat chiroptic effects of said chiral compounds, such small changes inthe relative concentrations can be readily and accurately detected.Starting from pure 1-enantiomer, a mixture of, for example, 49.5 mol %1-enantiomer and 50.5 mol % d-enantiomer is formed by the writing withcircularly polarized light. Starting from pure d-enantiomer, a mixtureof, for example, 50.5 mol % 1-enantiomer and 49.5 mol % d-enantiomer isobtained by writing with circularly polarized light having a directionof rotation which is opposite to that mentioned above.

An example of an inherently dissymmetric olefinic chromophore which issuitable for an optical recording medium according to the invention isformed by the class of fluorenes, a representative of which is shown inFIG. 2. In said formula, Y represents a O- or S-atom or a NH- orN-alkyl- or SO- or a CH₂ - or SO₂ -group, A represents hydrogen or analkoxy group, R represents Hydrogen or an alkyl group and B representshydrogen or an alkyl or alkoxy group. R and A can also occupy othersubstitution positions and may be present at the benzene ring more thanonce or they may be present at different benzene rings. BothB-substituents are identical and can also take up other substitutionpositions, such that the "lower half" of the molecule is symmetrical. Asuitable alkyl group is the methyl group; a suitable alkoxy group is themethoxy group. In this connection, it is noted that the aromatic groupsof the "upper half" and "lower half" of the molecules are not bothsituated in the plane of the drawing because of the "twist" in themolecule. During the photochemical switching with left-handed orright-handed circularly polarized light, the aromatic groups move awayfrom each other and through the plane of the drawing, so that, forexample, the 1-form is isomerized into the d-form. The conversion of the1-form into the d-form is reversible.

Another suitable compound for an optical recording medium according tothe invention is a (thio)xanthone as shown in FIG. 3. In said Figure, Xrepresents a S- or O-atom or a NH- or N-alkyl- or SO- or a SO₂ -groupand Y, A, R and B have the above mentioned meaning.

When the substituents B of the compounds shown in FIGS. 2 and 3 differfrom each other or take up different substitution positions, compoundsas shown in FIGS. 4 and 5 are formed. In said compounds, B1≠B2 and B1and B2 represent a hydrogen or alkyl- or alkoxy groups. A suitable alkylgroup is the methyl group; a suitable alkoxy group is the methoxy group.X, Y, A and R have the above-mentioned meaning. In such compounds, the"lower half" is non-symmetrical, as a result of which cis-isomers andtrans-isomers exist in addition to 1- and d-enantiomers. Exposure tolight of a first wavelength λ1 causes the cis-isomer to be partiallyconverted into one (i.e. 1- or d-) of trans-isomers. Said conversion canbe detected by the rotation of the plane of polarization of linearlypolarized light. Since the cis- and trans-isomers have a differentabsorption maximum, the trans-isomer can be converted into the originalcis-isomer by means of a second wavelength λ2. In this embodiment, aninscribable and erasable optical recording medium is obtained by usingtwo wavelengths and without carrying out an additional heating step.

A suitable embodiment of an optical recording medium according to theinvention is characterized in that the recording layer comprises aracemic cis-mixture or a racemic trans-mixture of a compound as shown inFIGS. 4 or 5. When, for example, a racemic cis-mixture is exposed tocircularly polarized light of a first wavelength, a trans-mixture withan 1% excess of 1-enantiomer is formed, i.e. approximately 50.5 mol % oftrans-1-enantiomer and 49.5 mol % of trans-d-enantiomer. This is calledan enriched trans-mixture. This small excess of trans-1-enantiomer canbe detected with linearly polarized light. The recorded information iserased by unpolarized light of a second wavelength, which secondwavelength coincides with the absorption maximum of the trans-mixture.Starting from an enriched cis-mixture an analogous mechanism applies.

Another suitable compound for an optical recording medium according tothe invention, which compound may be subject to acis-transisomerisation, is shown in FIG. 1a. In said Figure, A, X and Rhave the above meaning. The corresponding trans-isomer is shown in FIG.1b. Said compounds belong to the class of bisphenanthrenes, arepresentative of which is described in an article by B. Feringa et al.,in J. Am. Chem. Soc., 99, 602-603 (1977).

A racemic cis- or trans-mixture of a compound as shown in FIGS. 1, 4 or5, can be prepared by mixing such a compound with a suitable,light-absorbing dye which heats up upon exposure to light. A racemiccis-mixture is formed from the cis-compound; a racemic trans-mixture isformed from the trans-compound.

A further suitable embodiment of an optical recording medium accordingto the invention is characterized in that the recording layer comprisesa compound as shown in FIG. 6. In said Figure, X and Y have theabove-mentioned meaning and R is hydrogen, an alkyl or alkoxy group.When R is hydrogen, the "lower half" of the molecule is symmetrical andonly 1-d isomerization takes place.

For the sake of simplicity, the compounds shown in FIGS. 1, 2, 3, 4, 5and 6 are drawn as if they extend in a flat plane. In reality, however,said compounds have a "twist" in the olefinic π-bond.

The substituents have an influence on the wavelength at which saidphoto-isomerization takes place. To shift the absorption maxima of saidcompounds towards longer wavelengths, use can be made of suitablecombinations of electron-donating and electron-attracting groups, theso-called donor and acceptor substituents. Absorption maxima at longerwavelengths have the advantage that solid state diode lasers emitting inthe wavelength range of 770-840 nm or about 670 nm can be used. Anothermuch used wavelength is 633 nm, i.e. the HeNe laser wavelength. Thedonor and acceptor substituents may be located on different sides of thedouble bond. They may alternatively be located on the same side of thedouble bond. Examples of donor substituents are NH₂, N(CH₃)₂, OCH, SCH₃and all the other donor substituents which are known to those skilled inthe art. Examples of suitable acceptor substituents are NO₂, N₂ ⁺, SO₂CH₃, CN, 1,2,2 tricyano ethylene, 2,2 dicyano ethylene and all theacceptor substituents which are known to those skilled in the art.

Such a substituent can also act as a barrier against thermalracemization of an inscribed enantiomer mixture. In the case of thermalracemization the aromate groups of the optically active compounds movepast each other and through the plane of the drawing. Other substituentscan provide a better bonding to a polymer matrix of the record layer inwhich the optically active compounds are dissolved or dispersed.Suitable substituents are, for example, the hydroxy group or an acidgroup when polymethyl (meth)acrylate is used as the matrix. Therecording layer consisting of a polymer matrix in which the opticallyactive compounds are dissolved or dispersed can be provided on asuitable carrier by means of known methods, such as spin coating. Byproviding the optically active compounds with (meth)acrylatesubstituents, whether or not via spacer groups such as -(CH₂)_(n) - and-(CH₂ CH₂ O)_(n) -, the compounds can be co-polymerized with the polymer(meth)acrylate matrix. In this manner, the concentration of opticallyactive compounds can be increased because the degree of solubility ofsuch compounds in PMMA (polymethyl methacrylates) is generally low.

The polymer matrix may alternatively be composed of other polymers, suchas polystyrene and diethylene glycol bisallyl carbonate (availablecommercially as CR-39).

A glass plate can be used as the carrier, but it is alternativelypossible to use quartz and transparent synthetic resin substrates, suchas polycarbonate, polystyrene, PMMA and CR-39.

The optically active compounds can also be provided on the carrierwithout a matrix by means of known techniques, such as spin coating.

A light-absorbing dye which heats up upon exposure may additionally beadded to the optically active compounds to enhance the racemization ofone of the enantiomers. The dye may alternatively be provided in aseparate layer between the carrier and the record layer. Suitable dyescan be selected from the group which includes squarylium, methine,pyrylium, cyanine and naphthaquinone.

It is obvious that instead of one polymer layer also two or more polymerlayers can be applied to the carrier, each layer comprising saidoptically active compounds, such that each layer comprises compoundswhich are sensitive to a different wavelength(range). In this manner,multilayer "recording" can take place, which results in an increasedinformation density of the recording medium.

The object of providing a method of writing, reading and erasinginformation, according to a new principle, in an optical recordingmedium according to the invention, is achieved by a method which ischaracterized in that a racemic mixture of the inherently dissymmetricolefinic chromophore is used, which mixture is enriched with a 1- ord-enantiomer during the writing of information with circularly polarizedlight, and linearly polarization light is used for reading, the rotationof the plane of polarization of the linearly polarized light beingdetected and linearly or circularly polarized light having a directionof rotation which is opposite to that used for writing being used forerasing. As described above, in principle, i.e. so long as no cis-transisomerisation occurs, the writing, reading and erasing of informationcan be carried out at the same wavelength; only the polarization stateof the light used has to be varied. As mentioned above, already at a lowconcentration, said inherently dissymmetric olefinic chromophoresexhibit sufficiently great chiroptic effects to detect a readablesignal.

A further embodiment of the method according to the invention ischaracterized in that during the writing of information with circularlypolarized light, an enantiomer of the inherently dissymmetric olefinicchromophore is converted into a non-racemic mixture, i.e. a mixturecomprising an excess of one of the two enantiomers (either 1- or d-), inwhich embodiment linearly polarized light is used during reading, therotation of the plane of polarization of the linearly polarized lightbeing detected and linearly or circularly polarized light having adirection of rotation which is opposite to that used for writing beingused for erasing.

Starting from a racemic mixture, i.e. 50 mol % of l-enantiomer and 50mol % of d-enantiomer, an excess of one of the enantiomers, for example50.5 mol % of 1-enantiomer and 49.5 mol % of d-enantiomer, may be formedduring the writing with circularly polarized light. By virtue of thegreat chiroptic effects of said compounds, the excess can be detected bymeans of the rotation of the plane of polarization of linearly polarizedlight. During erasing a racemic mixture is formed and inscribing can beresumed. This process can be repeated many times.

The above methods can be carried out with chiral compounds as shown inFIGS. 2 and 3. By virtue of the symmetry of the "lower half" of thesecompounds, the cis-trans isomerization is equivalent to 1-disomerization.

Another embodiment of the method according to the invention ischaracterized in that during the writing of information with light of afirst wavelength, the inherently dissymmetric olefinic chromophore issubject to a cis-trans-isomerization, and linearly polarized light isused for reading, the rotation of the plane of polarization of thelinearly polarized light being detected and light of a second wavelengthbeing used for erasing, the chromophore being subject to atrans-cis-isomerization. A cis-compound as shown in FIGS. 1a, 4, 5 and 6can be converted into the corresponding trans-compound by exposure tolight. (See for example, FIG. 1b). This cis-trans-isomerization does notrequire polarized light. The cis- and trans-compounds have a differentspectral absorption maximum, thus permitting the trans-compound to bereconverted into the corresponding cis-compound with light of adifferent wavelength. This principle enables a write and erase process.Since the cis- and trans-isomers have a different molar rotation, thereading operation can be carried out with linearly polarised light. Inthis embodiment, two wavelengths of the light used are necessary,however, an additional heating step during erasing is not required. Inthis method, the reading of information can alternatively take place bymeasuring the absorption of light, because the cis- and trans-isomershave a different spectral absorption maximum.

In the above-described methods, it is possible to erase the informationwhich has been inscribed. In a number of applications this is notnecessary or even undesirable. An alternative method is characterized inthat an enantiomer of the inherently dissymmetric olefinic chromophoreis racemized by means of light during the writing of information, andlinearly polarized light is used for reading, the rotation of the planeof polarization of the linearly polarized light being detected. Startingfrom a 1- or d-enantiomer, a racemic mixture is obtained during writing.When this mixture cannot readily be enriched with one of theenantiomers, a non-erasable optical recording medium is obtained.Whether enriching is possible or not depends on, inter alia, the type ofsubstituent of the optically active compound. This method can be carriedout using chiral compounds as shown in FIGS. 2 and 3, 1-d isomerisationtaking place in said method.

A further embodiment of the method according to the invention ischaracterized in that the recording medium comprises a dye having anabsorption maximum which corresponds to the wavelength of the light tobe used, an enantiomer of the inherently dissymmetric olefinicchromophore being racemized by means of said light during the writing ofinformation, and linearly polarized light being used for reading. Thelight-absorbing dye may be contained in a separate layer or in therecording layer. The dye can be selected from the above-mentioned groupof dyes. Exposure to light brings about a generation of heat as a resultof which thermal racemization takes place. This method can be carriedout with compounds as shown in FIGS. 1, 4, 5 and 6. The absorptionmaximum of the dye can be selected such that at said wavelength nocis-trans-isomerization takes place.

The invention also relates to novel compounds which are suitable for usein an optical recording medium and method according to the invention.These compounds are shown in FIGS. 2, 3, 4, 5 and 6, where X, Y, A, R,B, B1 and B2 have the above-mentioned meaning. More specifically,compounds preferred for use herein are inherently dissymmetric olefinicchromophores having the formula ##STR1## and/or inherently dissymmetricolefinic chromophores selected from the group of compounds including l-and d- eantiomers, isomers and mixtures thereof having the generalformula: ##STR2## wherein: Z is selected from ##STR3## and wherein inFormula (I) and (II): (1) A is hydrogen or an alkoxy group; (2) R ishydrogen or an alkyl group; (3) R₁ is hydrogen or an alkyl or alkoxygroup; (4) Y is an O, S, N-H, N-alkyl, SO, CH₂ or SO₂ group; (5) X is anO, S, N-H, N-alkyl, SO, or SO₂ group; (6) B, B₁ and B₂ are hydrogen oran alkyl or alkoxy group; (7) C and D are hydrogen or together form afused aromatic group; (8) R and A may be present more than once on thesame or different benzene ring; (9) the B substituents are the same, and(10) B₁ and B₂ may be the same or different.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained in greater detail by means of anexemplary embodiment and with reference to the accompanying drawings, inwhich

FIGS. 1a-1b represent the structural formulae of cis- and transisomersof a bisphenanthrene,

FIG. 2 represents the structural formula of a fluorene,

FIG. 3 represents the structural formula of a (thio)xanthone,

FIGS. 4, 5 and 6 represent the structural formulae of other compounds inaccordance with the invention, and

FIG. 7 diagrammatically shows a part of a cross-sectional view of anoptical recording medium according to the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In FIG. 7, reference numeral 1 diagrammatically represents a part of across-sectional view of an optical recording medium. A quartz-glasscarrier 3 having a thickness of 1.2 mm and a diameter of 12 cm isprovided with a recording layer 5, having a thickness of 1 μm, by meansof spin coating. For this purpose, a 10 wt. % solution of PMMA inchlorobenzene is prepared, in which also 4 wt. % of the compound shownin FIG. 1a is dissolved, in which compound X is CH₂ and R and A arehydrogen. The recording medium is ready after evaporation of thechlorobenzene. A laser-light source having a wavelength of 300 nm isused for writing the information, the recording frequency being 0.5 MHzand the linear velocity of the recording medium being 1.2 m/s. Acis-trans-isomerization takes place in the exposed areas, i.e. thecompound having the structural formula shown in FIG. 1a is convertedinto that shown in FIG. 1b. The reading of the recorded informationtakes place with linearly polarized light having the same wavelength andhaving a low power. The plane of polarization in the exposed areasappears to have rotated through +0.1°. The recorded information iserased with light having a wavelength of 400 nm, which results intrans-cis isomerization, in said operation the plane of polarizationrotates through -0.1°.

EXAMPLE 2

Starting from a racemic mixture of a compound shown in FIG. 2, where A,B, R=H and Y=O, an optical recording medium is manufactured as describedin Example 1. The medium is inscribed with right-handed circularlypolarized light with λ=350 nm. One of the enantiomers is enriched in theexposed areas, such that the information can be read with linearlypolarized light of the same wavelength having a relatively low power of0.1 mW. The plane of polarization in the exposed areas appears to haverotated through 0.1°. The recorded information is erased with linearlypolarized light having a power of 10 mW, also at the same wavelength.

EXAMPLE 3

In accordance with Example 1, an optical recording medium ismanufactured having a cis-compound in accordance with FIG. 4 as thechiral compound, where Y is CH₂ ; A, R, and B₁ are hydrogen and B₂ isCH₃. A squarylium dye in a quantity of 1 wt. % is also dissolved in therecording layer. Upon exposure to light with λ=780 nm, a racemiccis-mixture is formed in the exposed areas. Detection takes place withlinearly polarized light of the same wavelength having a power of 0.1mW.

We claim:
 1. An optical recording medium comprising a carrier and arecording layer which contains at least one optically active organiccompound in the form of an inherently chiral dissymmetric olefinicchromophore that comprises enantiomers which rotate the plane ofpolarization of polarized light and upon exposure to light can beconverted at least partly from one form to another form.
 2. An opticalrecording medium as claimed in claim 1, wherein the inherentlydissymmetric olefinic chromophore is a compound having the formula##STR4## where Y represents a O-, S-, NH, N-alkyl, SO, CH₂ or SO₂-group, A represents hydrogen or an alkoxy group, R represents hydrogenor an alkyl group and B represents hydrogen, or an alkyl- or alkoxygroup.
 3. An optical recording medium as claimed in claim 1, wherein theinherently dissymmetric olefinic chromophore is a compound having theformula ##STR5## where X represents a O-, S-, NH, N-alkyl, SO or SO₂-group A represents hydrogen or an alkoxy group, R represents hydrogenor an alkyl group, Y represents a O-, S-, NH, N-alykl, SO, CH₂ - or SO₂-group and B represents hydrogen or an alkyl or alkoxy group.
 4. Anoptical recording medium as claimed in claim 1, wherein the inherentlydissymmetric olefinic chromophore is selected from compounds having thefollowing formulae: ##STR6## where X represents a -O, -S, -NH, N-alkyl,SO or -SO₂ group, A represents hydrogen or an alkoxy group, R representshydrogen or an alkyl group, Y represents a O-, S-, -NH, N-alkyl, SO, SO₂or -CH₂ group, R₁ represents hydrogen or an alkyl or alkoxy group, andB₁ and B₂ represents hydrogen or an alkyl- or alkoxy group, and -B₁ andB₂ are different.
 5. An optical recording medium as claimed in claim 1wherein the recording layer comprises a racemic mixture of theinherently dissymmetric chromophore.
 6. An optical recording medium asclaimed in claim 1 wherein the inherently dissymmetric olefinicchromophore is dissolved or dispersed in a polymer layer on the carrier.7. An optical recording medium as claimed in claim 1 wherein theinherently dissymmetric olefinic chromophore is covalently bonded to apolymer chain of a polymer layer on the carrier.
 8. An optical recordingmedium comprising a carrier and a recording layer which contains atleast one optically active organic compound inherently dissymmetricolefinic chromophore selected from compounds having the formula ##STR7##or inherently dissymmetric olefinic chromophores selected from the groupof compounds having the formula: ##STR8## wherein: Z is selected from##STR9## and wherein which in Formula (I) and (II): (1) A is hydrogen oran alkoxy group; (2) R is hydrogen or an alkyl group; (3) R₁ is hydrogenor an alkyl or alkoxy group; (4) Y is an O, S, N-H, N-alkyl, SO, CH₂ orSO₂ group; (5) X is an O, S, N-H, N-alkyl, SO, or SO₂ group; (6) B, B₁and B₂ are hydrogen or an alkyl or alkoxy group; (7) C and D arehydrogen or together form a fused aromatic group; (8) R and A may bepresent more than once on the same or different benzene ring; (9) The Bsubstituents are the same; and (10) B₁ and B₂ may be the same ordifferent.
 9. An optical recording medium as claimed in claim 8 whereinthe recording layer comprises a racemic mixture of the inherentlydissymmetric chromophore.
 10. An optical recording medium as claimed inclaim 8 wherein the inherently dissymmetric olefinic chromophore isdissolved or dispersed in a polymer layer on the carrier.
 11. An opticalrecording medium as claimed in claim 8 wherein the inherentlydissymmetric olefinic chromophore is covalently bonded to a polymerchain of a polymer layer on the carrier.